The present invention provides unique pressure actuated valves and related methods that may be used in applications where a plurality of fluid emission devices are provided on a plurality of lines containing fluid under pressure, in order to prevent fluid from escaping from the fluid emission devices until a consistent threshold or predetermined level of fluid pressure is obtained in the lines, without causing significant water pressure and/or flow loss through the valves. Valves of the present invention include a housing having an upper body and a lower body that may be coupled together to form an inner chamber, the lower body having an inlet in communication with a source of fluid under pressure, the upper body having an outlet in communication with a fluid emission device. A hollow movable fluid transmission piston is provided in the chamber for communicating between the inlet and outlet. The chamber formed between the upper and lower bodies may be provided with cup seals at opposite ends of the chamber to reduce friction and prevent fluid from entering therein, and a side air vent. A wider lower end of the piston is urged toward the lower housing body using a biasing mechanism, which pushes an O-ring at the lower end against a stop to prevent fluid from entering into the hollow center of the piston. When fluid with a sufficient pressure is provided from the source, it pushes the O-ring away from the stop and against the urging of the biasing member, allowing fluid to flow around the stop, through the center of the piston, and out through the upper body into the fluid emission device. The vent in the chamber allows air inside the chamber (surrounding the biasing mechanism) to escape so that there is no added air pressure within the chamber to be overcome by the fluid pressure.